This invention relates to printed wiring board design and, more particularly, to a method for shielding copper signal lines from radiating electromagnetic energy to neighbouring signals.
The past two decades has seen major advances in the performance, size and cost of electronics equipment for all types of end-product applications. Underlying these improvements has been the rapid increase in sophistication of the two most custom elements of an electronics product, namely integrated circuits and printed wiring boards. The printed wiring board no longer serves as the passive interconnection panel that it used to be. In addition to providing for component mounting and interconnection, it has assumed a major role in determining the active functioning of electronic circuits.
In response to the increasing demands for cost-effectively maintaining integrated circuit performance, printed wiring boards have evolved into many different types of packaging and interconnecting structures. To complicate the design process, these printed circuit structures are being implemented with a wide variety and combination of materials and they can also be manufactured by several different processes.
The printed wiring board (PWB) is, in general, a layered dielectric structure with internal and external wiring that allows electronic components to be mechanically supported and electrically connected internally to each other and to the outside environment. Printed wiring boards are the most commonly used packaging medium for electronic circuits and systems.
Copper-clad laminate and prepreg are the basic building blocks of the printed wiring board. Prepreg refers to fabric impregnated with resin in which the resin is partially reacted so that it has the correct properties for use in subsequent operations, whether that be the manufacture of laminates or for use as the bonding sheet for the production of multilayer boards. One or more pieces of prepreg with copper foil on the outside, are laminated under heat and pressure to form the copper-clad laminate. The copper is then patterned using resists and etched. At present, the copper-clad laminates and prepregs are made with a variety of different matrix resin systems and reinforcements. FR-4 laminates, for example, are constructed of multiple plies of epoxy-resin-impregnated woven glass cloth. The dimensional stability of the epoxy-fiberglass is adequate for its use with high density wiring and its availability in a semicured prepreg makes it particularly desirable for rigid multilayer applications. In fact, it is the most widely used material in the printed wiring board industry because its properties satisfy the electrical and mechanical needs of most applications.
The ever increasing packaging density and faster propagation speeds, which stem from the demand for high-performance systems, have forced the evolution of the boards from single-sided to double-sided to multilayer boards. The necessity of a controlled impedance for high-speed traces, the need for bypass capacitors and the need for low inductance values for the power and ground distribution networks have made the requirement of power and ground planes a must in high performance boards. These planes are obviously only possible in multilayer construction.
A printed wiring board is, therefore, generally a composite of organic and inorganic dielectric material with multiple layers. The interconnects or the wires in these layers are connected by xe2x80x98viaxe2x80x99 holes, which can be plated with metal to provide the electrical connections between respective layers. In addition to the ground and power planes, used to distribute bias voltages to the ICs and other discrete components, the signal lines are distributed among various layers to provide the interconnections in an optimum manner.
At low frequencies, a signal path on a printed wiring board may be represented electrically as a lumped network of series resistances and shunt capacitances. However, as the frequency is increased, this approach of lumped circuit modelling breaks down, and signal paths must be regarded as transmission lines. The commonly used PWB transmission line structures are microstrip, embedded microstrip, stripline and dual striplines. The microstrip configuration simply refers to the case where the printed wiring board conductor is separated from a reference plane, either ground or power, by a dielectric. The stripline configuration, on the other hand, has reference planes above and below the conductor. A typical multilayer board of more than two signal layers, then, may have both stripline and microstrip geometries.
In general, the properties of importance that need to be minimized for good printed board design are signal delay, distortion and xe2x80x98crosstalkxe2x80x99 noise. Crosstalk is a category of noise induced primarily by the electromagnetic coupling between signal lines. In printed wiring boards, crosstalk can occur by the electrical coupling between nearby signal traces in a given layer. Crosstalk decreases noise margins and degrades signal quality. This, of course, can be a major limiting factor in communication systems performance. Crosstalk increases with longer trace coupling distances, smaller separation between traces, shorter pulse rise and fall times, larger magnitude currents or voltages being switched, and decreases with the use of adjacent power and ground planes or with power and ground traces interlaced between signal traces on the same layer.
The printed wiring board is an essential part of a total electronic circuit packaging system. As more and more functions are integrated on a chip, more connections off the chip are required, and more circuit traces are needed to interconnect them. The need for high density has led to finer conductor lines and closer spacing. With the closeness of the conductors, and higher signal speeds, the coupling of signals into adjacent conductor lines becomes greater and introduces noise and false signals into systems.
Two types of signal coupling determine the amount of crosstalk in a circuit: inductive coupling and capacitive coupling. These two types of coupling decrease with increasing distance between source and receiver. Most crosstalk can be attributed to adjacent wires. Because parallel and adjacent wires on a printed wiring board layer interact both capacitively and inductively, the distance over which adjacent wires are parallel should be minimised. To minimise the crosstalk caused by capacitive coupling, high frequency designs should also incorporate ground planes under each signal layer. Ground planes virtually eliminate the crosstalk caused by capacitive coupling between adjacent layers. Ideally, then, crosstalk between neighboring signals can be reduced by maximizing signal-to-signal spacing and by minimizing signal-to-ground distances.
However, as mentioned, crosstalk is also a problem at high frequencies because, as operating frequencies increase, signal wavelengths become comparable to the length of some of the interconnections on the printed wiring board. Under these conditions and depending on the degree of inductive and capacitive coupling, the interconnections may actually become antennas and begin broadcasting.
These factors, plus a host of others, contain many interdependencies and are often at odds with one another. For example, high wiring density is required to minimize interconnect delays as well as size, cost and weight. However, as signal lines are placed closer together, their mutual coupling increases, with a corresponding rise in crosstalk levels.
Therefore, the design of PWBs has become quite a challenging task, especially when designing high-performance and high-density boards. Most significantly, electromagnetic coupling between the interconnects (signal traces) is the primary factor that sets the upper limit to the interconnect density.
The invention involves the creation of a metal shield around a copper signal line or differential pair in a printed wiring board to isolate the signal(s) from radiating energy to neighboring signals, thus eliminating totally any potential crosstalks. The metal shield is built from a microstrip or stripline configuration in the PWB by scribing, laser ablating, scoring, chemical etching, photolithographic developing, mechanical milling, other chemical and mechanical means of cutting grooves or microchannels from the surface at both sides of the signal trace through the dielectric material (FR4, teflon, getek or any other suitable dielectric) to the ground plane, exposing the ground metal all along the bottom of the channel. Metallisation is then applied to the surface and the grooves (side walls and bottom) by electroless plating, electroplating, immersion plating, chemical vapour deposition, screen printing conductive paste or by other similar processes.
The inventive technique results in a complete shield around the copper signal trace(s) providing for shielding comparable to that of a coaxial cable. This invention is vital for the reliable performance of printed wiring boards at high frequencies. The invention mitigates the deleterious effects caused by the ever-increasing interconnection (signal trace) density of high density and high speed boards and will enable transmissions at speeds greater than 10 GHz with existing copper technology.
The invention, then, is the utilization of printed wiring board fabrication techniques in conjunction with other suitable technologies to form metal shields around copper signal traces in high-speed, high-density boards, thereby eliminating any potential crosstalks. Since the method described involves the use of existing equipment and technologies, an obvious advantage of the invention is that its implementation should not make much difference to the overall system cost. As such, the invention is likely to be critical to the performance of future high-speed products.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings.